Accurate determination of water content is an important aspect of most vadose zone monitoring programs. A common method for measuring soil water content in unsaturated soils is time domain reflectometry (TDR). In this method, the TDR technique involves the measurement of the propagation velocity of a high frequency signal transmitted along a probe. Since the dielectric constant of water (˜80) is significantly higher than most soils (2–7), measurements of the velocity of propagation can be used to determine the soil volumetric water content. TDR probes are typically made with two or three metal rods that function as waveguides placed parallel to each other, inserted into the soil at the point where the measurements are to be made.
The use of TDR probes for measuring water content in natural undisturbed soil in the unsaturated zone between the land surface and the water table (the vadose zone) is often limited by the structure, depth and condition of the soil. In shallow soil, TDR probes inserted into the ground from the land surface create only minor disturbance of the natural soil properties. Inserting TDR probes into deep soil horizons can be problematic however, when a natural undisturbed soil condition is required. Standard TDR probes can be installed into deep levels, but this usually involves (1) excavating a deep trench followed by inserting the probes horizontally through the trench walls to the deeper soil horizons, or (2) drilling a wide diameter vertical borehole and using a special instrument to insert the TDR probe through the borehole wall into the surrounding soil.
Both the trench and wide diameter vertical borehole techniques are undesirable because they can significantly disturb the natural soil properties in the near-field environment where the TDR probes are installed. This disturbance may create preferential flow paths for water to bypass the natural soil material and affect the representativeness of the TDR reading. Even if the excavated trench or wide diameter borehole are backfilled, soil disturbances caused by those methods can create preferential flow paths for water and affect the representativeness of the TDR readings. When standard TDR probes are installed into undisturbed rocky or pebbly material, they are susceptible to bending, crimping or complete failure because they loose their parallel orientation. Deformation of the probes away from the parallel configuration can bias the readings and create uncertainties about the accuracy of the water content measurements. Stiff rods bearing TDR probes can be pushed into the ground under very high loads, but those rods are only built to be inserted into shallow layers and only work well in very soft soils.
Prior techniques have also used non-invasive TDR probes attached to the surface of the examined material. Those stiff acrylic pads bearing coiled waveguides allowed moisture measurements to be made only on relatively smooth and flat surfaces. These and other limitations of existing TDR probe installation techniques in the deep soils has led me to develop a new apparatus and method suitable for the measurement of the soil dielectric constant or permittivity and electrical conductivity in the deep vadose zone. The embodiments of the present invention allow multi level installation of soil moisture probes, which are based on dielectric constant or pemittivity measurements, to any desired depth from land surface with only minimal, if any, disturbance of the natural properties of the soil column.